Conventionally, the present inventors have developed a blood flow rate measuring device that irradiates with a laser beam a body tissue having blood cells such as an eye ground and a skin, leads random speckled patterns, i.e. speckle images, which are formed by interference of reflected light from the blood cells, onto an image sensor such as a solid-state image sensor (CCD or CMOS), sequentially captures and stores a large number of these speckle images at predetermined time intervals, selects a predetermined number of images from the large number of the stored images, calculates a value reflecting a time variation speed of output in each pixel of each image and then calculates the velocity of blood cells (blood flow rate) from the value.
In this kind of blood flow rate measuring device, the output variation speed of each pixel corresponds to a movement velocity of blood cells. Accordingly, a blood flow distribution in a body tissue can also be color-displayed on a monitor screen as a two-dimensional image (blood flow map) based on the value of the output variation calculated in each pixel. A blood flow map observed actually is composed of sequential blood flow maps calculated so as to become about 30 scenes per second. Hereinafter, the sequential blood flow maps are sometimes called as an original map. The blood flow map can be displayed as an animation. Therefore, this device has been put to practical use as a device for inspecting hemodynamics of an eye ground or skin (see Japanese Examined Patent Publication No. Hei 5-28133, Japanese Examined Patent Publication No. Hei 5-28134, Japanese Unexamined Patent Publication No. Hei 4-242628, Japanese Unexamined Patent Publication No. Hei 8-112262, Japanese Unexamined Patent Publication No. 2003-164431, and Japanese Unexamined Patent Publication No. 2003-180641).
In addition, the present inventors proposed a blood flow rate imaging device that analyzes a blood flow variation appearing at regular intervals synchronously with cardiac beats in each site within an observation field on the basis of sequential blood flow maps obtained between a few seconds in blood flow measurement, introduces a numerical value capable of distinguishing an arterial site with a steep rise waveform and a venous site with a waveform gradually going up and down, i.e., skewness, and displays an arterial pulse part and a venous pulse part on a blood flow map (see WO2008/69062 Pamphlet).
However, when the blood flow rate imaging device proposed by the present inventors is used to detect, for example, a blood flow of an eye ground, sequential original maps obtained by calculating a blood flow distribution actually are generally grainy as shown in FIG. 1 and outlines of blood vessels are composed of grains. These grains arise from facts that a speckle image for calculating a blood flow value intrinsically has much noise and a statistical error occurs since samples for measuring blood flow of each pixel are limited in number. There are essential differences on location and size of grains between a series of original maps. In other words, numerical values representing a blood flow rate in each pixel vary considerably in respective maps. It is known that a blood flow in an arterial blood vessel on a retina changes periodically by cardinal beats. A numerical value or a waveform thereof includes important information on a peripheral circulatory function. However, in order to detect the value or the waveform accurately, it is necessary to distinguish accurately whether each pixel of an original map obtained at a certain time is positioned in the retina vessel part in the surface layer or is positioned in the blood flow of the peripheral choroid and other tissues (background blood flow). Such distinction is difficult to perform on the basis of the original map with rough grains as shown in FIG. 1. Therefore, it has been a big problem to develop a method for distinguishing region of blood vessel running on a surface accurately, i.e., the retina vessel region, from a region of background blood flow.
Further, when distribution of skewness characterizing a blood flow waveform of an eye ground is required using the conventional blood flow rate imaging device as proposed in the WO2008/69062 Pamphlet, blood flow values in numerous pixels adjacent to a target pixel are taken into calculation so as to increase samples in number and reduce a statistical error. However, since the blood flow values are calculated based on pixels extracted from a region containing both a blood vessel part and a background tissue part that have a different waveform from each other, information on the blood flow waveforms are confused mutually. Consequently, when the distribution of skewness is displayed as an image, there is a problem that a waveform of a thin arterial blood vessel is difficult to distinguish, since it is buried in a waveform of tissue blood flow existing in the background. A waveform of a target blood vessel can be obtained by extracting a blood flow value along a course of a blood vessel and specifying skewness. As stated above, however, there has been a problem that it is difficult to find a method for extracting only blood vessels.
Further, using the conventional blood flow imaging device, a region called a rectangular rubber band such as a rectangular portion illustrated at the upper part of the center in FIG. 1 is set up manually along a course of a predetermined blood vessel in a blood flow map as shown in FIG. 1, and then, a blood flow waveform in the rubber band (refer to FIG. 2) or a distribution of blood flow rate in cross-section of blood vessel (refer to FIG. 3) is examined. The horizontal axis of FIG. 3 shows pixels. In such a method, it is necessary to select a straight blood vessel and, for examining a blood flow waveform, it is also necessary to set up the rubber band in an elongated form and exactly so as to match to the width of the blood vessel. For examining a cross-section of blood vessel, a straight blood vessel is also selected and a bit broader region in parallel to the blood vessel is set up, then, blood flow values are averaged in a direction of the blood vessel running to obtain a cross-sectional view of velocity distribution as shown in FIG. 3.
However, since most retina vessels are not straight as shown in FIG. 1, only limited blood vessels can be measured by the method setting up the rectangular region. In addition, very complicated work is required to set accurately a rectangular region on a blood vessel every time blood flow analysis is performed.
It has been considered that a relation between a diameter of arterial blood vessel and a blood flow waveform includes very important information for understanding not only an ocular disease but also systemic hemodynamics. Therefore, if a retina vessel can be selected freely to measure a blood flow waveform and an effective diameter thereof, it is of great significance. Accordingly, it has been a great problem to develop a method in which a meandering blood vessel can be analyzed as well as a straight blood vessel.
On the other hand, when a blood flow waveform in each site of an eye ground (observation region) is converted to a numerical value using skewness, the skewness is affected with a secondary peak and fluctuation of peak position in the waveform if they exist. Further, arterial blood flow waveforms upon rising and going down may relate to different factors regarding a peripheral circulatory function, respectively. Therefore, it is insufficient to characterize a blood flow waveform by only skewness, and thus it is also necessary to introduce other indices so as to judge holistically.